Daytime running lights augment vehicle safety by enhancing the visibility of a vehicle having daytime running lights. Thus, daytime running lights are becoming a more common feature on vehicles. Because of the recognized importance of daytime running lights on vehicles, Canada has legislated to require daytime running lights on vehicles that are sold in Canada.
In daytime running lights on a vehicle, a high-power output switching transistor delivers current to a high wattage daytime running light bulb filament on the vehicle. Referring to FIG. 1, a high-power MOSFET (Metal Oxide Semiconductor Field Effect Transistor) 102 is used to deliver current from a power source 104 to the filament of a daytime running light 106. The intensity (and thus the brightness) of the daytime running light is determined by the duty cycle of a switched DC voltage, from a voltage switching circuit 108, applied to the gate of the MOSFET 102.
The MOSFET 102 is shown as an example field effect switching device. A field effect transistor is preferred as the switching device rather than a BJT (Bipolar Junction Transistor) because of the higher switching speed and lower resistance of a field effect transistor when the field effect transistor is turned on.
The power source 104 is typically from a battery system of the vehicle, and the battery system includes a fuse which blows and open-circuits the drain of the MOSFET 102 from the power source 104 when excessive current flows though such a fuse. However, a partial short load 110 may be coupled to the source of the MOSFET 102. The partial short load 110 has an impedance which is not low enough to blow the fuse of the battery system. Nevertheless, the impedance of the partial short load 110 may be low enough to cause high current to flow through the switching MOSFET 102. Such high current flowing through the switching MOSFET 102 results in damage to the switching MOSFET 102.
Prior art protection circuits prevent damage to a field effect switching transistor from a short load. However, the prior art protection circuits use relatively complicated circuitry having relatively numerous components. Such complicated circuitry may add higher costs and higher possibility of malfunction if one of the components were to become inoperative. For example, U.S. Pat. No. 4,750,079 to Fay et al. uses digital circuitry to remove an enable signal from a MOSFET, U.S. Pat. No. 5,272,392 to Wong et al. uses a feedback mechanism to maintain a constant level of current through a MOSFET, U.S. Pat. No. 5,390,069 to Marshall uses a current mirror in a protection circuit for limiting current through a MOSFET, and U.S. Pat. No. 5,694,282 to Yockey uses a microprocessor for limiting current through a MOSFET.
In addition, the prior art protection circuits, including U.S. Pat. No. 4,595,966 to Huber et al. and U.S. Pat. No. 5,438,237 to Mullins et al., do not include a latching circuit to keep the switching transistor 102 turned off once the current through the switching transistor 102 reaches an excessive level. Without the latching circuit, the daytime running light may flash on and off as the current through the switching transistor alternately increases and decreases from the mechanism of the prior art protection circuit. Such flashing on and off of the daytime running light may be a traffic hazard on the road and also may cause further degradation of the switching transistor 102.